For transmitting short messages by radio, particularly messages containing personal paging calls, it is usual to use a large number of radio transmitters, each with a limited range. Such transmitters are adapted for synchronous radio transmission, i.e. all of them send the same message with the same frequency. The transmission is of the binary frequency modulation (frequency shift keying, FSK)type. The transmitters are further adapted for sending the message bits simultaneously. In known installations for sending personal paging calls the method of transmission is normally:
Transmission on a line of a message from a central station to all radio stations simultaneously; and
transmission of the message by radio with differences in propagation time on different lines first being compensated, so that the message is transmitted simultaneously from all radio transmitters.
An example of a system for nation-wide transmission of personal paging calls is described in "Final Report of the Brittish Post Office Code Standardisation Advisory Group (POCSAG)", London 1978. A method of providing simultaneousness in the transmission of the message with use of time signals sent by broadcasting is also described in EP-A-0042144.
When the same message is sent by radio from several transmitters simultaneously, it is unavoidable that some receivers will receive the transmission from two radio transmitters. If the radio transmitters have exactly the same frequency, their field strengths may be combined to an increased field strength and good reception obtained, but in another place approximately a quarter wavelength away, their field strengths can cancel each other so that reception is made impossible. The disadvantage of fading field strength in certain places, standing waves, is mitigated by the frequencies of two adjacent transmitters being given a small offset. Instead of quite zones, beats will then occur with the frequency difference, which can be of the order of magnitude 500 Hz, while the nominal frequency may be 150 MHz, for example. The beats affect the ability of receiving the separate binary characters in the message, for which reason the bit frequency in the transmission should not exceed the beat frequency.
The true carrier frequency of the transmitters may deviate from the selected frequency by 50 Hz at most. The frequency stability requirement is thus high, and it has so far been met by using high-stability transmitters of by transmitting signals on a radio link for synchronizing the carrier frequency of the transmitters. Both methods result in expensive installations.
In a receiver which is situated such that the transmission from two transmitters is received in it, the separate characters must arrive simultaneously, or otherwise there will be uncertainty as to when the character begins and ends. It is considered that the uncertain part of a character should not exceed 20% of the character length, and with a character rate of for example 512 bits/s applicable for the mentioned POCSAG system the uncertainty may be a maximum of 250 micro-seconds.
Radio receivers for the reception of coded personal paging calls are described, inter alia, in the U.S. Pat. No. 3,835,394.